1. Technical Field
This invention relates generally to voltage-controlled oscillators (VCOs), and more particularly to converting a voltage to a current for use in a current-controlled oscillator (ICO) in which process, voltage, and temperature variations can be compensated independently.
2. Discussion of Background Art
Voltage controlled oscillators using Complementary Metal-Oxide Semiconductor (CMOS) technologies are known to be greatly affected by process, voltage supply, and temperature variations. U.S. Pat. No. 5,061,907 to Rasmussen and U.S. Pat. No. 5,331,295 to Jelinek et al. are directed to achieving automatic compensation for process, voltage, and temperature effects.
Rasmussen discloses a VCO that includes a multistage ring oscillator, a voltage-to-current converter, process compensation circuitry, and a trip-point compensation circuit. The voltage-to-current converter linearly converts an input signal to an output current signal that is independent of varying process and temperature effects on the transistor in the ring oscillator. The converter thus provides a variable control current. The process compensation circuitry responds to the tuning voltage input signal to provide a current dump output signal. However, this signal is dependent on the transistor strength, which in turn depends on the transistor fabrication process, supply voltage, and operating temperature. The trip-point compensation circuit subtracts the process compensation current from the voltage-to-current converter generated current to produce a net current.
Jelinek et al. discloses a VCO including a multistage ring oscillator, a first current source, a second current source, and an attenuator. The first current source provides a substantially constant current independent of process, supply voltage, and temperature. The second current source provides a current that varies in response to process, supply voltage, and temperature. Both current sources generate respective current signals independent of the input signal to the VCO. The attenuator, responsive to the VCO's input voltage signal, provides to the ring oscillator a control current signal which determines the frequency of oscillation of the ring oscillator.
Neither Rasmussen nor Jelinek et al. can control or regulate the quantity of process, voltage supply, and temperature compensation, or adjust each of these variables independently. Such ability is a common need in VCO designs.